The invention relates to a process and apparatus for the manufacture of silicon rods having a columnar structure comprising monocrystalline crystal zones with a crystallographic preferred orientation. More particularly, it relates to such a process and apparatus wherein molten silicon is transferred in a continuous or non-continuous manner from a silicon reservoir to a crystallization chamber wherein controlled solidification of the silicon occurs while maintaining a supernatant pool of molten material and with the solidified silicon being withdrawn by means of a downwardly movable support.
In this process, which is known from U.S. Pat. No. 4,312,700, there is, however, close and long contact between the pool of molten material or the solidifying silicon and the adjacent vessel wall, which is generally made of graphite, and optionally includes a thin layer of a protective melt which has been applied to the vessel. This contact results in a concentration of undesired impurities, such as, for example, carbon, in the edge zone of the silicon rod that has been withdrawn. The resulting contaminated material is of a lower quality than material that is free of impurities and, therefore, solar cells produced therefrom, for example, have a distinctly reduced efficiency. Hence, it is often necessary to remove the contaminated edge zones of the silicon rod in order to obtain a solar cell base material that meets the specified requirements.
Accordingly, it is an object of the invention to provide a process and apparatus for producing silicon rods that are especially suitable as solar cell base material and are of higher purity and quality than previously obtained according to the prior art.